A typical automotive vehicle is steered by transmitting operations of a manually steerable device, such as a steering wheel, to a steering mechanism for directing the road wheels. Generally, the manually steerable device is located inside the vehicle passenger compartment, and the steerable road wheels are located at the front of the vehicle. Thus, a suitable steering mechanism is necessary to couple the manually steerable device and the road wheels.
In order to overcome limitations presented by mechanical steering systems, it has been proposed to utilize a steering system in which the manually steerable device is not mechanically coupled to the road wheels and steering movement is achieved by an electrically controlled motor, a so-called steer-by-wire system.
In a steer-by-wire system, a road wheel motor actuator, connected to the road wheels, operates in response to a control command generated by a road wheel controller. The road wheel controller receives various measurements and estimation signals such as steering wheel angle, road wheel angle, and vehicle speed, and sends a control command to the actuator in order to make the road wheels follow the steering wheel command. In a steer-by-wire system, there is no mechanical connection between the road wheel actuation system and the steering wheel, although both road wheels may be mechanically linked together to move in unison.
It has also been proposed to utilize a steer-by-wire system in which the two front road wheels are steered independently. In this type of steer-by-wire system, the two front road wheels are not mechanically coupled. Instead, the two independent actuators actuate the two road wheels independently.
As an electro-mechanical system, driven by electrical motors, the road wheel actuation steering mechanism of a steer-by-wire system is subject to the influence of friction in the form of a force or a torque. For example, a typical steer-by-wire system includes a pair of road wheels which each of them is driven by a ball screw via a DC brush-less motor. There are many sources of friction in such a system, including the ball screw bearings, the interface between the screw and the tie-rod, and the load and side load caused by the road wheels. The total friction force experienced by the steer-by-wire system is the sum of the respective frictions resident in the system. The resultant friction is highly nonlinear and may adversely affect the performance of the steering control system resulting in steady state errors, limit cycles, and stick-slip motion. Consequently, the road wheels may not follow the steering wheel command as desired.
Therefore, the friction force must be compensated for in order to ensure that the road wheels follow the steering wheel input command, especially at slow steering rate inputs. Although the friction acting on the motor actuator may be reduced through improved mechanical hardware design, there are, however, cost and space constraints associated with this solution.
Accordingly, the present invention utilizes control system methodologies to compensate and overcome the effects of friction present in a steer-by-wire system. Specifically, the present invention utilizes system modeling, estimation, and control methodologies to compensate the effects of friction in steer-by-wire systems.
A friction compensator in the steer-by-wire control system produces a friction compensating torque value equal and opposite in sign to the instantaneous friction torque. This compensating friction torque is added to the system control signal to eliminate the effects of friction present in the system. The friction compensator produces the compensating friction torque according to one of two schemes: model-based or non-model based. The model-based scheme is based on a suitable friction model that captures the behavior of the friction to compensate the friction torque. The non-model based scheme does not depend on models of friction to compensate the friction torque.
In the present invention invention, several model-based and non-model based friction compensation schemes are described. The model-based scheme encompasses a number of different methods including a standard model-based scheme, a disturbance torque observer-based scheme, an adaptive friction compensation scheme, or a model reference adaptive control scheme. The non-model based scheme includes a fuzzy logic scheme.
Although this invention describes friction compensation in a steer-by-wire system with two independent actuator-driven road wheels, it is not limited to such a steering system. The present invention is adaptable for use in any steer-by-wire or electromechanical steering system.